The capacity for wireless communication systems has gone through dramatic increase during the last few years, and it is foreseen that this growth will continue in the future. In order to meet the increasing capacity requirement for the next generation systems, as specified by International Telecommunications Union-Radio Communication Sector (ITU-R), the 3rd Generation Partnership Project (3GPP) is now evolving the current Long Term Evolution (LTE) system towards the advanced version, the so called “LTE-Advanced”.
LTE-Advanced requires a bandwidth of up to 100 MHz, which is much wider than 20 MHz of the current LTE system. The current spectrum utilization pattern excludes the possibility of assigning a contiguous wideband. Therefore, carrier aggregation (CA) of individual component carriers (CCs) has been proposed. This leads to a multi-carrier transmission system. In order to maintain backward compatibility, independent layer-1 (physical layer) transmission can be used, which is in coherence with the LTE Rel'8 assumptions on each CC, as proposed e.g. in 3GPP TR 36.814 v1.0.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA Physical layer aspects,” February, 2009.
With independent layer-1 transmission on each CC, feedback of a channel quality information (e.g. Channel Quality Indicator (CQI)) and an acknowledgement e.g. Ack/Nack) per CC may be required, as proposed e.g. in 3GPP TS 36.213 v8.6.0, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures,” March, 2009.
However, such uplink signaling may contribute to a large amount of transmission overhead. Moreover, when multiple-input multiple-output (MIMO) transmission is used, the overhead is further increased because of the requirement for signaling additional control information (e.g. a Precoding Matrix Indicator (PMI) and/or a Rank Indication (RI). In addition to inefficiency caused by heave uplink overhead, some cell-edge users that are power limited cannot support the uplink transmission over multiple CCs and will thus be in an outage situation. A problem is basically how to achieve better downlink performance for a multi-carrier system, while still trying to limit the uplink feedback overhead from CQI and Ack/Nack's.